There is no restriction on everyone else starting threads, too, of course. It's just that the official volunteers will be committing themselves to a particular level of participation, and occassionally to bringing up topics that people may be interested in but hesitant to start themselves.
And of course sticking around and monitoring/replying/prodding the discussion in new directions rather than taking off in the middle of the thread if it all gets sticky and difficult for example.

This (obviously) isn't a scheduled thing, so long as there's four new threads or so started each week? I found with my chats that I struggle to manage a 'scheduled' thing each week, given the way my life is headed right now, yet I can usually still find time to keep up-to-date here and so on.
If someone volunteered, are the hosts understanding of certain weeks where it might be literally impossible for them to post due to exams, and the like a few times each year, provided they generally managed to post 4 or so new items each week (or more)?